MEDIUM NC#37 Blind #45 Linked List

143. Reorder List

📖 Problem

You are given the head of a singly linked-list. The list can be represented as: L0 → L1 → ... → Ln-1 → Ln Reorder the list to be on the following form: L0 → Ln → L1 → Ln-1 → L2 → Ln-2 → ... You may not modify the values in the list's nodes. Only nodes themselves may be changed.

🧠 Visual Learning Aid

1 Model the input into the right structure
2 Choose the core technique and invariant
3 Execute step-by-step with a sample
4 Validate complexity and edge cases

JS/TS Refreshers

  • •Array methods (`push`, `pop`, `shift`, `slice`)
  • •Object/Map/Set usage patterns
  • •Function parameter and return typing
  • •Midpoint overflow-safe math
  • •Loop invariants
  • •Monotonic condition design

Logical Thinking Concepts

  • •Define invariants before coding
  • •Check edge cases first (`[]`, single element, duplicates)
  • •Estimate time/space before implementation
  • •Apply Binary Search reasoning pattern

💡 Approach

  • → Find middle of list using fast/slow pointers
  • → Reverse second half of list
  • → Merge first half and reversed second half alternately
  • → Time: O(n), Space: O(1)

🧭 Prerequisites

#19 Remove Nth Node From End of List #138 Copy List with Random Pointer

🛠️ Hints & Pitfalls

Hints

  • •Find middle of list using fast/slow pointers
  • •Reverse second half of list
  • •Merge first half and reversed second half alternately

Common Pitfalls

  • •Time: O(n), Space: O(1)

🧪 Test Cases

Test Case 1
Not run
Input:
reorderList(list1);
Expected:
null
Test Case 2
Not run
Input:
reorderList(list2);
Expected:
null
Test Case 3
Not run
Input:
reorderList(head: ListNode | null);
Expected:
Computed from hidden reference

📝 Code Editor

📚 Reference Solution

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🔗 Related Problems

#2 Add Two Numbers #19 Remove Nth Node From End of List #21 Merge Two Sorted Lists #23 Merge k Sorted Lists #25 Reverse Nodes in k-Group
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